Method of producing titanium alloy composite material

ABSTRACT

A method of producing a titanium alloy composite material comprises mixing carbon fibers and a powder of an element which forms a carbide in reaction with carbon, subliming the element under high temperature vacuum, and coating the carbon fibers with a layer containing the element and the carbide to produce coated carbon fibers. The method further comprises mixing the coated carbon fibers and titanium alloy powder to form a mixture, and applying a mechanical impact force to the mixture to fix the carbon fibers on the surface of the titanium alloy powder to obtain a carbon fiber-fixed titanium alloy powder. The method further comprises sintering the carbon fiber-fixed titanium alloy powder to form a sintered body and plastic working the sintered body to disperse the carbon fibers in crystal grains of the titanium alloy.

TECHNICAL FIELD

The present invention relates to a titanium alloy composite material, amethod of producing the titanium alloy composite material, a titaniumclad material using the titanium alloy composite material, and a methodof producing the titanium clad material.

BACKGROUND ART

Titanium alloys have high relative strength and excellent corrosionresistance, and have mainly been used in the fields of aerospace, deepsea exploration, chemical plants, and the like. Recently, titaniumalloys have been widely used for consumer uses such as heads or shaftsof golf clubs, components of watches or fishing goods, and eyeglassframes.

Recently, composite materials containing a titanium alloy and carbonfiber combined for further improving mechanical properties such astensile strength and toughness have been proposed. For example, PatentDocuments 1 and 2 each disclose an automobile component formed of atitanium alloy containing carbon fibers such as carbon nanofibers.Patent Documents 1 and 2 each further describe injecting ions of oxygen(O), nitrogen (N), chlorine (Cl), chromium (Cr), carbon (C), boron (B),titanium (Ti), molybdenum (Mo), phosphorus (P), aluminum (Al), and thelike into the carbon nanofibers, to thereby improve wetness andadhesiveness between the carbon nanofibers and metal. Further, puretitanium has also been cladded to a side surface of a core material madeof a titanium alloy, for example, for obtaining functions and propertiesthat cannot be obtained with a single substance (see Patent Document 3,for example).

Patent Document 1: JP 2004-225084

Patent Document 2: JP 2004-225765

Patent Document 3: JP 2002-000971

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The inventors of the present invention, after diligent study, have foundthat in the conventional techniques disclosed in Patent Documents 1 and2, titanium and carbon fibers react with each other during formation ofa composite. Thus, the inventors of the present invention have foundthat the original properties of the carbon fibers as a reinforcingmaterial are significantly degraded, and mechanical strength as expectedcannot actually be obtained. Further, as described in theabove-mentioned Patent Documents, it is also found that use of carbonnanofibers subjected to ion injection treatment as a carbon fiber hasimproved dispersibility of the carbon nanofibers in an alloy, however,reactivity of the carbon nanofibers with titanium is rather accelerated,and mechanical strength of the carbon nanofibers is somewhat reduced. Inthe conventional technique disclosed in Patent Document 3, mechanicalproperties of both a titanium alloy and pure titanium are originally notsufficient, and thus cladding of the titanium alloy and pure titaniumprovides no clad material having remarkably improved mechanicalproperties.

Therefore, the present invention has been made in view of solving theproblems described above, and an object of the present invention is toprovide a titanium alloy composite material having excellent mechanicalstrength such as tensile strength, Young's modulus, toughness andhardness.

Another object of the present invention is to provide a titanium cladmaterial having remarkably improved mechanical properties such astensile strength, elongation and fracture toughness.

Means for Solving the Problems

The inventors of the present invention, after conducting intensivestudies and development for solving the conventional problems describedabove, have found that dispersion of carbon fibers coated with a layercontaining an element which forms carbide in reaction with carbon andthe carbide formed thereby in crystal grains of titanium alloy iseffective for solving the problems, to complete the present invention.Further, the inventors of the present invention have found that a cladmaterial obtained by cladding this titanium alloy composite material anda titanium alloy having a high fracture toughness has remarkablyimproved mechanical properties such as tensile strength, elongation andfracture toughness.

That is, a titanium alloy composite material according to the presentinvention is characterized by dispersing carbon fibers coated with alayer containing an element which forms carbide in reaction with carbonand the carbide formed thereby in crystal grains of the titanium alloy.

It is preferable that the element which forms carbide in reaction withcarbon include at least one selected from the group consisting ofsilicon (Si), chromium (Cr), titanium (Ti), vanadium (V), tantalum (Ta),molybdenum (Mo), zirconium (Zr), boron (B) and calcium (Ca).

It is preferable that the carbon fibers include carbon nanotubes,vapor-grown carbon fibers or a mixture thereof. The titanium alloycomposite material preferably comprises 0.1% to 10% by mass of thecarbon fibers. The layer preferably has a thickness of at least 0.5 nm.

A method of producing a titanium alloy composite material according tothe present invention is characterized by comprising: a step of mixingcarbon fibers and powder of an element which forms carbide in reactionwith carbon, and then sublimating the element under high temperaturevacuum to coat the carbon fibers with a layer containing the element andthe carbide; a step of mixing the carbon fibers obtained in the formerstep and titanium alloy powder, and applying mechanical impact force onthe mixture to fix the carbon fiber on a surface of the titanium alloypowder; a step of sintering the carbon fiber-fixed titanium alloy powderobtained in the former step; and a step of plastic working the sinteredbody obtained in the former step to disperse the carbon fiber in crystalgrains of the titanium alloy.

It is preferable that a method of producing a titanium alloy compositematerial further comprises a step of aging the plastic-worked titaniumalloy composite material. The sintering is preferably conducted by apulse electric current sintering method. The plastic working ispreferably conducted by at least one process selected from a hot rollingprocess and an isothermal forging process.

The titanium clad material according to the present invention ischaracterized in that a titanium alloy composite material with carbonfibers coated with a layer containing an element which forms carbide inreaction with carbon and the carbide formed thereby dispersed in crystalgrains of the titanium alloy, and a titanium alloy having a higherfracture toughness than that of the titanium alloy composite materialare sinter bonded to one another. Further, it is preferable that thetitanium clad material comprise a pair of sheet materials made of thetitanium alloy having a higher fracture toughness than that of theabove-mentioned titanium alloy composite material, and a core materialmade of the above-mentioned titanium alloy composite material arrangedbetween the sheet materials. The core material preferably has ahoneycomb structure.

A method of producing a titanium clad material according to the presentinvention is characterized by comprising: laminating in a mold atitanium alloy composite material with carbon fibers coated with a layercontaining an element which forms carbide in reaction with carbon andthe carbide formed thereby dispersed in crystal grains of the titaniumalloy, and a titanium alloy having a higher fracture toughness than thatof the titanium alloy composite material; and sinter bonding thetitanium alloy composite material and the titanium alloy to one anotherby a pulse electric current sintering method.

Effect of the Invention

According to the present invention, a titanium alloy composite materialhaving excellent mechanical strength such as tensile strength, Young'smodulus, toughness and hardness, and a method of producing the same canbe provided.

Further, according to the present invention, a titanium clad materialhaving remarkably improved mechanical properties such as tensilestrength, elongation and fracture toughness, and a method of producingthe same can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A flow chart explaining a method of producing a titanium alloycomposite material of the present invention.

[FIG. 2] Diagrams showing examples of a laminate structure of a titaniumclad material of the present invention.

[FIG. 3] A diagram showing an example of the most preferred laminatestructure of the titanium clad material of the present invention.

[FIG. 4] Graphs showing results of X-ray diffraction measurement ofcarbon nanotubes coated with Si of Example 1.

[FIG. 5] An ultrahigh resolution FE-SEM image of titanium alloy powdercontaining carbon fibers fixed thereon of Example 1.

[FIG. 6] A metallographic microscopic image of a metallographicstructure of a sintered body of Example 1.

[FIG. 7] A metallographic microscopic image of a metallographicstructure of a titanium alloy composite material obtained in Example 1.

[FIG. 8] A graph showing results of strength measurement of materialsobtained in Examples 1 and 2 and Comparative Example 2 and 4.

[FIG. 9] Cutaway views of the titanium alloy composite material obtainedin Example 1 after material strength measurement.

[FIG. 10] Graphs showing results of X-ray diffraction measurement ofcarbon nanotubes coated with Cr of Example 4.

[FIG. 11] A metallographic microscopic image of a vicinity of a sinterbonded interface of a titanium clad material of Example 6.

[FIG. 12] An enlarged image of part A of FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail.

(Titanium Alloy Composite Material)

A titanium alloy composite material of the present invention is obtainedby dispersing carbon fibers coated with a layer containing an elementwhich forms carbide in reaction with carbon and the carbide formedthereby in crystal grains of the titanium alloy. That is, the layercoating the carbon fibers is formed of the carbide formed through apartial reaction between the element and the carbon fibers, and anunreacted element. This layer serves as a layer for suppressingreactions between the carbon fibers and titanium during formation of acomposite and improves wetness with the titanium alloy, and thusproperties of the carbon fibers as a reinforcing material are maintainedafter formation of the composite. In the present invention, such coatedcarbon fibers are dispersed in crystal grains, to thereby significantlyimprove mechanical strength such as tensile strength, Young's modulus,toughness and hardness. In the present invention, a state in which thecarbon fibers are dispersed in crystal grains of the titanium alloyrefers to a state in which the carbon fibers are at least partlyincorporated in fine crystal grains of the titanium alloy while moderatedispersibility is maintained through plastic flow during plasticworking.

Meanwhile, the inventors of the present invention have confirmed that,in the case where the coated carbon fibers are not dispersed in thecrystal grains, sufficient mechanical strength cannot be obtained with atitanium alloy composite material prepared by mixing coated carbonfibers and titanium alloy powder and then sintering the mixture. Themechanical strength is presumably reduced because the carbon fibers orTiC as carbide of the carbon fibers forms a brittle layer having a highhardness at a titanium alloy crystalline interface, and the brittlelayer having a high hardness serves as a defect causing cracks.

The fiber diameter, fiber length, shape, and the like of the carbonfibers of the present invention are not particularly limited, and aconventionally known carbon fiber generally used as a reinforcingmaterial can be used without limitation. Of those, carbon nanotubes, avapor-grown carbon fiber, or a mixture thereof is preferably used fromthe viewpoint of further improving the mechanical properties. Examplesof carbon nanotubes include monolayer carbon nanotubes and multilayercarbon nanotubes each formed by a vapor phase growth method, an arcdischarge method, a laser vaporization method, or the like. Examples ofvapor-grown carbon fibers include discontinuous carbon fibers obtainedthrough crystal growth in a vapor phase by a vapor phase growth method,and a graphite fiber. The vapor-grown carbon fibers may have any shapesuch as acicular, coiled, tubular, or cup, and two or more kinds thereofmay be blended. From the viewpoint of further improving the propertiesof a reinforcing material and the dispersibility in a titanium alloy,the carbon nanotubes preferably have a fiber diameter of 2 nm to 80 nmand a fiber length of 1 μm to 100 μm, and the vapor-grown carbon fiberspreferably have a fiber diameter of 80 nm to 200 nm and a fiber lengthof 5 μm to 100 μm.

The fiber diameter, fiber length, and shape of the carbon fibers in thetitanium composite material can be measured through structuralobservation with an ultrahigh resolution FE-SEM or a transmissionelectron microscope.

The content of the carbon fibers is preferably 0.1% to 10% by mass, morepreferably 0.2% to 5.0% by mass, and most preferably 0.4% to 3.0% bymass with respect to the titanium alloy composite material. The contentof the carbon fibers within the above ranges allows further improvementin mechanical properties.

Note that the content of the carbon fibers in the titanium compositematerial can be measured through structural observation with anultrahigh resolution FE-SEM or a transmission electron microscope, andelemental analysis and analysis in accordance with “JIS H1617 Methodsfor determination of carbon in titanium and titanium alloys”.

In the present invention, the element coating the carbon fibers is notparticularly limited as long as the element is capable of formingcarbide in reaction with carbon. The element is preferably at least oneselected from the group consisting of silicon (Si), chromium (Cr),titanium (Ti), vanadium (V), tantalum (Ta), molybdenum (Mo), zirconium(Zr), boron (B) and calcium (Ca). The element is more preferably atleast one selected from silicon (Si) and chromium (Cr). The elementsexemplified are capable of further improving the mechanical propertiesbecause the carbide of the elements has excellent compatibility with thetitanium alloy.

The thickness of the layer containing the above-mentioned element andthe carbide of the element is preferably at least 0.5 nm, morepreferably 2 nm to 50 nm from the viewpoint of further improving themechanical strength by dispersion enhancement into the titanium alloy,and particularly preferably 0.5 nm to 10 nm in the case where carbonnanotubes are used as the carbon fiber.

Note that structural observation with an ultrahigh resolution FE-SEM ora transmission electron microscope can confirm whether or not the carbonfiber is coated with the layer containing the element and the carbide ofthe element.

The titanium alloy to be used for preparation of the titanium alloycomposite material may have any crystal structure such as: anα-structure (such as Ti—O or Ti-5Al-2.5Sn); a near α-structure (such asTi-6Al-5Zr-0.5Mo-0.2Si, Ti-5.5Al-3.5Sn-3Zr-0.3Mo-1Nb-0.3Si,Ti-8Al-1Mo-1V, or Ti-6Al-2Sn-4Zr-2Mo); an α+β-structure (such asTi-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, or Ti-4.5Al-3V-2Mo-2Fe); anear β-structure (such as Ti-5Al-2Sn-2Zr-4Mo-4Cr or Ti-10V-2Fe-3Al); ora β-structure (such as Ti-15Mo-5Zr-3Al, Ti-11.5Mo-6Zr-4.5Sn,Ti-15V-3Cr-3Al-3Sn, Ti-15Mo-5Zr, or Ti-13V-11Cr-3Al). Further, atitanium alloy (e.g., a titanium alloy containing Ti-15V-6Cr-4Al as abase and TiB and/or TiC added in a small amount, or a titanium alloycontaining Ti-22V-4Al as a base and TiB and/or TiC added in a smallamount) containing fine particles of TiB and/or TiC dispersed in a metalstructure and disclosed in JP-A-2005-76052 can preferably be used. Inconsideration of the mechanical strength of the titanium alloy compositematerial to be obtained eventually, preferred examples of the titaniumalloy include Ti-6Al-4V, Ti-15Mo-5Zr-3Al, Ti-15V-3Cr-3Al-3Sn,Ti-10V-2Fe-3Al, Ti-4.5Al-3V-2Mo-2Fe, and a titanium alloy disclosed inJP-A-2005-76052.

(Method of Producing Titanium Alloy Composite Material)

Next, a method of producing the titanium alloy composite material of thepresent invention will be described.

FIG. 1 is a flow chart explaining a method of producing the titaniumalloy composite material of the present invention. This method ofproducing the titanium alloy composite material of the present inventionis characterized by including: a carbon fiber coating step of coatingcarbon fibers with a layer containing an element which forms carbide inreaction with carbon and the carbide formed thereby; a carbon fiberfixing step of fixing the carbon fibers on a surface of titanium alloypowder; a sintering step of sintering the carbon fiber-fixed titaniumalloy powder; and a carbon fiber dispersing step of dispersing thecarbon fiber in crystal grains of titanium alloy.

(1) Carbon Fiber Coating Step

The carbon fiber coating step of the present invention refers to a stepof coating the carbon fibers with the layer containing an element whichforms carbide in reaction with carbon and the carbide formed thereby. Inthis step, the carbon fibers and powder formed of the element whichforms carbide in reaction with carbon are charged into a mixing vesselprovided with a stirring mixer or the like, and the whole is mixed forabout 15 to 30 minutes. The carbon fiber may employ the same carbonfiber as that exemplified in the description of the titanium alloycomposite material. The powder to be used only needs to be formed of theelement which forms carbide in reaction with carbon, and is formed of atleast one selected from the group consisting of silicon (Si), chromium(Cr), titanium (Ti), vanadium (V), tantalum (Ta), molybdenum (Mo),zirconium (Zr), boron (B) and calcium (Ca). The particle shape andaverage particle size of the powder are not particularly limited, butuse of powder having an average particle size of 10 μm to 50 μm allowsfurther improvement in dispersibility of the carbon fiber.

Next, the mixture taken out of the mixing vessel is filled in anunsealed vessel allowing air flow between inside and outside of theunsealed vessel. The unsealed vessel is placed in a vacuum furnaceprovided with a sealed furnace body, heating means for heating insidethe sealed furnace body and a vacuum pump for creating vacuum inside thefurnace body. Then, inside of the furnace body is heated by heatingmeans while the inside of the furnace body is maintained in a vacuumstate with the vacuum pump, to thereby sublimate the powder of theelement which forms carbide in reaction with carbon. The vapor isbrought into contact with the carbon fibers to form a layer covering thesurface of the carbon fibers. This layer is made of the carbide formedin reaction between a part of the sublimated element and the carbonfiber, and an unreacted element. The conditions such as degree ofvacuum, heating temperature, and heating time may arbitrarily be set inaccordance with the kind of powder to be used. However, in considerationof a balance between production cost and quality of the layer coveringthe surface of the carbon fibers, the conditions preferably include adegree of vacuum of 1×10⁻² Pa to 1×10⁻³ Pa, a heating temperature of1,200° C. to 1,500° C., and a heating time of 5 hours to 10 hours, forexample. Temperature increase rate and temperature decrease rate are notparticularly limited, but are each preferably 100° C./h to 200° C./h.

In this way, the carbon fibers are coated with the element, to therebysuppress a reaction between the carbon fibers and titanium duringformation of a composite of the carbon fibers and the titanium alloy.

(2) Carbon Fiber Fixing Step

The carbon fiber fixing step of the present invention refers to a stepof fixing the carbon fibers obtained in the carbon fiber coating stepdescribed above on the surface of titanium alloy powder. In this step,the carbon fibers obtained in the carbon fiber coating step are mixedwith the titanium alloy powder. The mixing ratio of the carbon fibers tothe titanium alloy powder is not particularly limited. However, from theviewpoint of further improving the mechanical properties of the titaniumalloy serving as a base material, the mixture preferably includes 0.1%to 10% by mass, more preferably 0.2% to 3.0% by mass, and mostpreferably 0.4% to 1.0% by mass of the carbon fibers. The particle shapeand average particle size of the titanium alloy powder are notparticularly limited, but use of powder having an average particle sizeof 10 μm to 50 μm allows further improvement in mechanical properties ofa composite titanium alloy. In the case where the carbon fiber isincluded in the mixture in an amount of 3% or more by mass, titaniumalloy powder having a small average particle size is preferably usedfrom the viewpoint of suppressing aggregation of the carbon fibers.

Next, mechanical impact force is applied to the mixture of the carbonfibers and the titanium alloy powder to fix the carbon fiber on thesurface of the titanium alloy powder. In this way, release of the carbonfibers from the surface of a titanium alloy powder particle isprevented, and a homogeneous sintered body can be obtained in thesintering step described below.

Specific examples of means for applying mechanical impact force include:a stirring device such as a hybridization system providing highmechanical impact force (manufactured by Nara Machinery Co., Ltd.) or amechanofusion system (manufactured by Hosokawamicron Corporation); adispersing device employing medium particles; and a dry mixing andstirring device such as a Henschel mixer or a V-type mixer. Of those,the hybridization system capable of applying mechanical impact forceincluding shear force between a rotor and a stator, impact force betweenparticles, and impact force between a particle and a wall of the devicein a high speed flow is preferably employed for fixing the carbon fiberon the surface of the titanium alloy powder particle uniformly andrigidly.

(3) Sintering Step

The sintering step of the present invention refers to a step of heatingand sintering the carbon fiber-fixed titanium alloy powder obtained inthe carbon fiber fixing step described above. In this step, the carbonfiber-fixed titanium alloy powder obtained in the carbon fiber fixingstep is formed into a molded product as required, and sintering themolded product by a sintering method conventionally known in thetechnical field such as a pulse electric current sintering method, a hotpress method, a gas pressure sintering method, or a hot isotropicsintering method preferably in vacuum or in an inert gas atmosphere. Inthe conventional method, titanium and most of the carbon fibers reactwith each other during sintering. Meanwhile, in the sintering step ofthe present invention, the reaction between the carbon fibers andtitanium is suppressed by the layer covering the carbon fibers (thecarbon fibers partly reacts with titanium to form titanium carbide), andthe properties of the carbon fiber as a reinforcing material aremaintained.

Sintering conditions such as sintering temperature and sintering timemay arbitrarily be set in accordance with the sintering method to beemployed or the kind of titanium alloy to be used, and the conditionspreferably include a sintering temperature of 800° C. to 1,300° C. and asintering time of 5 minutes to 2 hours, for example.

Of the sintering methods exemplified above, the pulse electric currentsintering method is preferably employed from the viewpoint of obtaininga homogeneous sintered body simply in a short sintering time. In thecase where sintering is conducted by the pulse electric currentsintering method, the carbon fiber-fixed titanium alloy powder or themolded product thereof is filled in a graphite die, and the whole isheated to a temperature of 850° C. to 950° C. with a temperatureincrease rate of 50° C./min to 100° C./min, for example for, sinteringfor 5 minutes to 10 minutes in a degree of vacuum of 4.0 Pa under acompression load of 20 MPa to 30 MPa. In the sintering by the pulseelectric current sintering method, neck growth between particles aloneis accelerated, and coarsening of particles due to shrinkage betweenparticles hardly occurs. Thus, particle size before sintering isretained, and a sintered body having a fine structure is obtained. Inthis way, the sintered body has a fine structure, and thus the carbonfiber is easily dispersed in the crystal grains uniformly in the carbonfiber dispersing step described below. As a result, the mechanicalstrength of the titanium alloy composite material to be obtainedimproves.

(4) Carbon Fiber Dispersing Step

The carbon fiber dispersing step of the present invention refers to astep of plastic working the sintered body obtained in the sintering stepdescribed above for dispersing the carbon fibers in the crystal grainsof the titanium alloy. The plastic working may employ a methodconventionally known in the technical field without limitation, andexamples thereof include a rolling process, a forging process, and anextrusion process. Of those, the plastic working preferably employs atleast one process chosen from a hot rolling process and an isothermalforging process. In particular, the hot rolling process is preferredbecause the crystal grains are drawn into a form of fiber for furtherimproving the mechanical strength of the titanium alloy compositematerial.

In the case where the sintered body is subjected to plastic workingthrough the hot rolling process, rolling conditions such as rollingspeed, rolling temperature, and draft are not particularly limited.However, from the viewpoint of obtaining a titanium alloy compositematerial having excellent mechanical strength, the conditions preferablyinclude a rolling strain/pass of 0.1 to 0.2, a rolling temperature of700° C. to 850° C., and a draft of 65% or more. In particular, a draftof less than 65% may undesirably cause insufficient dispersion of thecarbon fiber in the crystal grain, and thus the mechanical strength ofthe titanium alloy composite material may degrade. Note that the term“draft” is defined by (h₁-h₂) ×100/h₁ (wherein: h₁ represents a sheetthickness before rolling; and h₂ represents a sheet thickness afterrolling).

In the case where the titanium alloy composite material is worked forproducing a product having axial symmetry such as a gear, working of asheet material may provide insufficient product precision due toin-plane anisotropy. Thus, it is preferred that a cylindrical sinteredbody be produced in the sintering step and the plastic working employ ahot extrusion process at preferably 1,000° C. or more and preferably1,000° C. to 1,100° C. or a swaging process.

(5) Aging Treatment Step

The method of producing a titanium alloy composite material of thepresent invention preferably further includes a step of subjecting thetitanium alloy composite material obtained in the carbon fiberdispersing step described above to aging treatment. Conditions for theaging treatment may arbitrarily be set in accordance with the kind oftitanium alloy serving as a base material, and the aging treatment maybe conducted at 400° C. to 600° C. for 4 h to 24 h, for example. Thetitanium alloy composite material is subjected to the aging treatment,to thereby further improve the mechanical strength of the titanium alloycomposite material.

(Titanium Clad Material)

A titanium clad material of the present invention is characterized inthat the titanium alloy composite material described above, that is, thetitanium alloy composite material dispersing carbon fibers coated with alayer containing an element which forms carbide in reaction with carbonand the carbide formed thereby in crystal grains of the titanium alloy,and a titanium alloy having a higher fracture toughness than that of thetitanium alloy composite material (hereinafter, abbreviated as hightoughness titanium alloy) are sinter bonded to one another.

FIG. 2 shows examples of a laminate structure of the titanium cladmaterial of the present invention. Examples of the laminate structure ofthe titanium clad material include: a structure (FIG. 2( a)) in which asheet material 2 formed of the high toughness titanium alloy is stackedon a sheet material 1 formed of the titanium alloy composite material toform a laminate, and the laminate is sinter bonded together; a structure(FIG. 2( b)) in which a sheet material 1 formed of the titanium alloycomposite material and a sheet material 2 formed of the high toughnesstitanium alloy are stacked alternatively to form a laminate, and thelaminate is sinter bonded together; a sandwich structure (FIG. 2( c)) inwhich a sheet core material 3 formed of the titanium alloy compositematerial is provided between a pair of sheet materials 2 each formed ofthe high toughness titanium alloy, that is, a sheet core material 3formed of the titanium alloy composite material is sandwiched by a pairof sheet materials 2 each faulted of the high toughness titanium alloyto form a laminate, and the laminate is sinter bonded together; asandwich structure (FIG. 2( d)) in which a sheet core material 4 formedof the high toughness titanium alloy is provided between a pair of sheetmaterials 1 each formed of the titanium alloy composite material, thatis, a sheet core material 4 formed of the high toughness titanium alloyis sandwiched by a pair of sheet materials 2 each formed of the titaniumalloy composite material to form a laminate, and the laminate is sinterbonded together; and a cylindrical structure (FIG. 2( e)) in which acylindrical core material 6 formed of the titanium alloy compositematerial is inserted into a cylindrical material 6 formed of the hightoughness titanium alloy to form a laminate, and the laminate is sinterbonded together. In the structure of FIGS. 2( c) or (d), the sheet corematerial 3 formed of the titanium alloy composite material or the sheetcore material 4 formed of the high toughness titanium alloy may have ahoneycomb structure for reduction in weight of the titanium cladmaterial. In consideration of a balance between the mechanicalproperties and the reduction in weight of the titanium clad material tobe obtained eventually, the most preferred structure is the structureshown in FIG. 3 in which: a plurality of sheet materials 7 each formedof the titanium alloy composite material and having a honeycombstructure are stacked together, and the whole is sandwiched by a pair ofsheet materials 1 each formed of the titanium alloy composite materialto form a honeycomb core material; the honeycomb core material issandwiched by a pair of sheet materials 2 each formed of the hightoughness titanium alloy, and the whole is sinter bonded together. Notethat in the structure described above employing the sheet materials 7each having a honeycomb structure, the pair of sheet materials 1 eachformed of the titanium alloy composite material may be omitted. In thelaminate structures described above, the size and thickness of the sheetmaterial, core material, and the like may arbitrarily be set inaccordance with a product. ps (High Toughness Titanium Alloy)

The high toughness titanium alloy to be used in the present invention isnot particularly limited as long as the high toughness titanium alloyhas a higher fracture toughness than that of the titanium alloycomposite material described above. To be specific, a high toughnesstitanium alloy having a higher fracture toughness than that of thetitanium alloy composite material may arbitrarily be selected fromtitanium alloys such as: an α-structure titanium alloy (such as Ti—O orTi-5Al-2.5Sn); a near α-structure titanium alloy (such asTi-6Al-5Zr-0.5Mo-0.2Si, Ti-5.5Al-3.5Sn-3Zr-0.3Mo-1Nb-0.3Si,Ti-8Al-1Mo-1V, or Ti-6Al-2Sn-4Zr-2Mo); an α+β-structure titanium alloy(such as Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, orTi-4.5Al-3V-2Mo-2Fe); a near β-structure titanium alloy (such asTi-5Al-2Sn-2Zr-4Mo-4Cr or Ti-10V-2Fe-3Al); a β-structure titanium alloy(such as Ti-15Mo-5Zr-3Al, Ti-11.5Mo-6Zr-4.5Sn, Ti-15V-3Cr-3Al-3Sn,Ti-15Mo-5Zr, or Ti-13V-11Cr-3Al); and a titanium alloy (e. g., atitanium alloy containing Ti-15V-6Cr-4Al as a base and TiB and/or TiCadded in a small amount, or a titanium alloy containing Ti-22V-4Al as abase and TiB and/or TiC added in a small amount) containing fineparticles of TiB and/or TiC dispersed in a metal structure and disclosedin JP-A-2005-76052. In consideration of the mechanical strength of thetitanium clad material to be obtained eventually, Ti-6Al-4V,Ti-15Mo-5Zr-3Al, Ti-15V-3Cr-3Al-3Sn, Ti-10V-2Fe-3Al,Ti-4.5Al-3V-2Mo-2Fe, and a titanium alloy disclosed in JP-A-2005-76052are preferred for excellent mechanical properties such as elongation andtensile strength. The high toughness titanium alloy may be subjected toknown solution aging treatment (e.g., subjecting the high toughnesstitanium alloy to solution treatment at 780° C. to 800° C. for 1 h, andthen to aging treatment at 400 to 500° C. for 10 to 30 h). The hightoughness titanium alloy is subjected to the solution aging treatment,to thereby enhance tensile strength of the high toughness titaniumalloy.

Note that the fracture toughness in the present invention is measured bya K_(IC) testing method in accordance with ASTM E399-90 or ISO 12737.

(Method of Producing Titanium Clad Material)

Next, a method of producing the titanium clad material of the presentinvention will be described.

A method of producing the clad material according to the presentinvention is characterized by laminating the titanium alloy compositematerial and high toughness titanium alloy described above in a mold,and sinter bonding the whole by a pulse electric current sinteringmethod.

To be specific, a sheet material (or core material) formed of thetitanium alloy composite material and a sheet material (or a corematerial) formed of the high toughness titanium alloy are arbitrarilylaminated in a die (graphite die), and the whole is heated to atemperature of 950° C. to 1,100° C. with a temperature increase rate of50° C./min to 100° C./min, for example, for sintering for 5 min to 10min in a degree of vacuum of 1.0 Pa to 4.0 Pa under a compression loadof 15 MPa to 30 MPa, to thereby bond together the sheet material foundof the titanium alloy composite material and the sheet material formedof the high toughness titanium alloy. In the sintering by the pulseelectric current sintering method, neck growth between particles aloneis accelerated, and coarsening of particles due to shrinkage betweenparticles barely occurs. Thus, particle size before sintering isretained, and a sinter bonded body having a fine structure is obtained.Thus, a titanium clad material having remarkably improved mechanicalproperties such as tensile strength, elongation, and fracture toughnesscan be obtained. Note that for enhancing bonding strength between thesheet material formed of the titanium alloy composite material and thesheet material formed of the high toughness titanium alloy, surfaces tobe bonded together are preferably subjected to surface treatmentconventionally known in the technical field such as degreasing treatment(e.g., washing with an organic solvent) or surface polishing treatment(e.g., polishing with #600 to #1000 sand paper) in advance.

The honeycomb core material formed of the titanium alloy compositematerial may be produced by: punching out hexagonal pieces from a sheetmaterial formed of the titanium alloy composite material with a laserpunch or the like, and removing flash obtained after punching asrequired to produce a sheet material formed of the titanium alloycomposite material and having a honeycomb structure; and subjectingsurfaces of the sheet materials formed of the titanium alloy compositematerial with a honeycomb structure that are to be bonded together tosurface treatment and degreasing treatment, stacking together the sheetmaterials with good precision by using an alignment jig or the like, andsandwiching the whole by a pair of sheet materials each formed of thetitanium alloy composite material. However, in the case where thetitanium clad material is produced by using the honeycomb core materialformed of the titanium alloy composite material, inside of the honeycombcore material is bonded in a state (e.g., a state of reduced pres sure)in accordance with conditions for pulse electric current sintering.Thus, in the case where the inside of the honeycomb core material mustbe adjusted to the same pressure as that of a use environment of thetitanium clad material, a minute vent hole may be provided on thehoneycomb core material.

(Application of Titanium Alloy Composite Material and Titanium CladMaterial)

The titanium alloy composite material and titanium clad material of thepresent invention have excellent mechanical properties such as tensilestrength, elongation, Young's modulus, fracture toughness, and hardness,and can be widely used for products requiring such properties includingindustrial machinery, automobiles, motorcycles, bicycles, householdappliances, aerospace equipment, ships and vessels, sports and leisureequipment, and medical equipment. To be specific, the titanium alloycomposite material and titanium clad material of the present inventionmay preferably be used: for connecting rods, engine valves, valvesprings, retainers, suspensions, body frames, or the like inapplications for automobiles and motorcycles; for fan blades, compressorblades, discs, frames, body panels, fasteners, flags, spoilers, maingears, exhaust air ducts, fuel tanks, or the like in applications foraerospace equipment; and for artificial bones, artificial joints,implant screws, surgical instruments, or the like in applications formedical equipment.

As sports and leisure equipment, in the case where the titanium alloycomposite material of the present invention is used for a face part of agolf club, for example, thickness reduction can be realized due torelative strength improvement compared with a conventional titaniumalloy, to thereby increase the coefficient of rebound. The thicknessreduction allows surplus weight, to thereby enhance the degree offreedom in design and allow setting of unprecedented centers of gravity.As described above, a golf club provided with a head employing thetitanium alloy composite material of the present invention can extendthe carrying distance and enlarge the sweet spot. Thus, a golfer can hita ball straight with little bend.

EXAMPLES

Hereinafter, the present invention will be described in more detail byway of examples and comparative examples, but the present invention isnot limited thereto.

Evaluation of mechanical properties of the titanium alloy compositematerial was conducted following the methods described below.

<Material Strength Measurement>

The titanium alloy composite material was cut out into a dumbbell-shapedtest piece having a length of 30 mm in a rolling direction, and paralleland perpendicular directions, the length of the parallel part being 15mm, and the width of the parallel part being 5 mm with a carbon dioxidegas laser. A strain gauge was attached to the parallel part, andstrength measurement was conducted at a crosshead speed of 1 mm/min byusing a material testing machine (manufactured by Shimadzu Corporation,Autograph AG-1, 100 kN).

<Young's Modulus Measurement>

Young's modulus measurement was conducted by using a modulus measuringdevice (manufactured by Toshiba Tungaloy Corporation, UMS-R).

<Hardness Measurement>

Hardness measurement was conducted by using a Rockwell hardness testingmachine (manufactured by Akashi Corporation, ATK-F3000).

Example 1

20 g of multilayer carbon nanotubes having an average fiber diameter of10 to 25 nm and an average fiber length of 10 to 50 μm and 2 g of Sipowder having an average particle size of 40 μm were weighed with anelectrical balance, and then were mixed in a mortar for about 30 min.The obtained mixture was charged into a 1-L tantalum vessel. A tantalumcap was placed over the container, and then the container was placed ina vacuum furnace. The vacuum furnace was heated from room temperature to300° C. in 4 hours under vacuum to a degree of vacuum of 2×10⁻³ Pa,heated to 1,400° C. in 7 hours, and maintained at 1,400° C. for 5 hoursfor sublimation of Si, to thereby coat a surface of the carbon nanotubeswith Si. The degree of vacuum while the temperature was maintained at1,400° C. was maintained at about 3×10⁻³ Pa by Si sublimation. Then, thefurnace was cooled under vacuum, to thereby obtain carbon nanotubescoated with Si. FIG. 4 shows results of X-ray diffraction measurement ofthe obtained carbon nanotubes. The results of X-ray diffractionmeasurement, and EDX analysis and observation with an ultrahighresolution field emission scanning electron microscope revealed that asurface modified layer (layer containing Si and SiC) having a thicknessof 0.5 nm in a thin position and about 5 nm in a thick position wasformed on the surface of the carbon nanotubes.

A Ti-6Al-4V alloy produced as titanium alloy powder by a powderatomization method and having a particle size distribution including2.3% by mass of +45 μm, 20.2% by mass of 38 to 45 μm, 27.8% by mass of25 to 38 μm, and 49.7% by mass of −25 μm was prepared. Carbon nanotubeswere weighed such that they were included in an amount of 0.5% by massin a mixture of this titanium alloy powder and the Si-coated carbonnanotubes obtained above. Mechanical impact force was applied to themixture in an argon gas by using a hybridizer (manufactured by NaraMachinery Co., Ltd.) which is a kind of powder stirring and mixingdevice. As shown in FIG. 5, the carbon nanotubes were attached to thesurface of the titanium alloy powder after the treatment. The carbonnanotubes attached to the surface of the titanium alloy powder werebeaten by collision of the titanium alloy powder and was embedded (i.e.,fixed) directly below the surface of the titanium alloy powder.

50 g of the raw material powder subjected to fixing treatment wasweighed and charged into a graphite die of a pulse electric currentsintering device. The raw material powder was pressurized at 30 MPa witha graphite cylinder, depressurized to a degree of vacuum on the order of4 Pa, heated from room temperature to 900° C. with a temperatureincrease rate of 100 ° C./min, and maintained at 900° C. for 5 min forsintering. The obtained sintered body (i.e., intermediate) was observedwith a metallographic microscope. As shown in FIG. 6, the sintered bodyhad a structure in which the carbon nanotubes and titanium carbideformed through a partial reaction between the carbon nanotubes andtitanium surrounded the titanium alloy fine particles.

Next, the sintered body was cut into a size of 35 mm×35 mm×5 mm, andsubjected to pack welding with a stainless steel SUS 304 sheet materialfor preventing oxidation during hot rolling. The cut-out piece washeated to about 800° C. by burner heating, and subjected to hot rollingin a longitudinal direction as a sheet material at a rolling strain/passof 0.1 and a draft of 68%, to thereby obtain a titanium alloy compositematerial of Example 1. The obtained titanium alloy composite materialwas observed with a metallographic microscope. As shown in FIG. 7, thetitanium alloy composite material had a structure in which the carbonnanotubes and titanium carbide were dispersed in the crystal grains ofthe titanium alloy.

FIG. 8 shows the results of material strength measurement of thetitanium alloy composite material of Example 1. FIG. 9 show results ofobservation of a broken-out section of the titanium alloy compositematerial after material strength measurement by using an ultrahighresolution field emission scanning electron microscope (manufactured byHitachi High-Technologies Corporation, S-5200) and an energy dispersiveX-ray analyzer (manufactured by EDAX Japan Co., Ltd.). In FIGS. 9( b) to(f), a light-colored part refers to a part containing a large amount ofa target element. FIG. 9 revealed that the shape of the carbon nanotubesremained and the carbon nanotubes near the surface was changed totitanium carbide through a reaction with titanium. Aluminum and vanadiumare components of the titanium alloy, but did not react with the carbonnanotubes. Coated Si was partly observed.

Table 1 collectively shows the results of measurement of tensilestrength, Young's modulus, and hardness.

TABLE 1 Tensile Young's strength modulus Hardness (MPa) (GPa) (HRC)Example 1 1500 126 47.8 Example 2 1614 127 49.0 Example 3 1522 125 46.0Example 4 1556 125 45.1 Example 5 1607 125 45.6 Comparative example 11074 109 39.3 Comparative example 2 963 110 37.9 Comparative example 3672 124 45.6 Comparative example 4 493 121 44.7

Example 2

The titanium alloy composite material was prepared in the same manner asin Example 1, and then a pack material was removed. The titanium alloycomposite material was charged into a vacuum furnace, subjected to avacuum, and subjected to an aging treatment at 500° C. for 8 hours underan argon gas (133 Pa) replacement, to thereby obtain the titanium alloycomposite material of Example 2. FIG. 8 shows the results of materialstrength measurement of the titanium alloy composite material of Example2. Table 1 collectively shows the results of measurement of tensilestrength, Young's modulus and hardness.

Example 3

The titanium alloy composite material of Example 3 was obtained in thesame manner as in Example 2 except that: the amount of the carbonnanotubes in the mixture of the titanium alloy powder and the Si-coatedcarbon nanotubes was changed to 0.4% by mass; and the draft of hotrolling was changed to 77%. Table 1 collectively shows the results ofmeasurement of tensile strength, Young's modulus, and hardness.

Example 4

20 g of multilayer carbon nanotubes having an average fiber diameter of10 to 25 nm and an average fiber length of 10 to 50 μm and 6 g of Crpowder having an average particle size of 10 μm were weighed with anelectrical balance, and then were mixed in a mortar for about 30 min.The obtained mixture was charged into a 1-L tantalum vessel. A tantalumcap was placed over the container, and then the container was placed ina vacuum furnace. The vacuum furnace was heated from room temperature to300° C. in 7 hours under vacuum to a degree of vacuum of 2×10⁻³ Pa,heated to 1,273° C. in 4 hours, and maintained at 1,273° C. for 5 hoursfor sublimation of Cr, to thereby coat the surface of the carbonnanotubes with Cr. The degree of vacuum while the temperature wasmaintained at 1,273° C. was maintained at about 3×10⁻³ Pa by Crsublimation. Then, the furnace was cooled under vacuum, to therebyobtain carbon nanotubes coated with Cr. FIG. 10 shows results of X-raydiffraction measurement of the obtained carbon nanotubes. The results ofX-ray diffraction measurement, and EDX analysis and observation with anultrahigh resolution field emission scanning micros cope revealed that asurf ace modified layer, which contains Cr, Cr₃C₂, and Cr₇C₃ and has athickness of 1 to 2 nm in a thin position and about 3 nm in a thickposition, was formed on the surface of the carbon nanotubes.

A Ti-6Al-4V alloy produced as titanium alloy powder by a powderatomization method and having a particle size distribution including2.3% by mass of +45 μm, 20.2% by mass of 38 to 45 μm, 27.8% by mass of25 to 38 μm, and 49.7% by mass of −25 μm was prepared. Carbon nanotubeswere weighed such that the carbon nanotubes were included in an amountof 0.4% by mass in a mixture of this titanium alloy powder and theCr-coated carbon nanotubes obtained above. Mechanical impact force wasapplied to the mixture in an argon gas by using a hybridizer(manufactured by Nara Machinery Co., Ltd.) which is a kind of powderstirring and mixing device, and the Cr-coated carbon nanotubes werefixed directly below the surface of the titanium alloy powder.

50 g of the above-mentioned raw material powder subjected to fixingtreatment was weighed and charged into a graphite die of the pulseelectric current sintering device. The raw material powder waspressurized at 30 MPa with a graphite cylinder, depressurized to adegree of vacuum on the order of 4 Pa, heated from room temperature to900° C. with a temperature increase rate of 100° C./min, and maintainedat 900° C. for 5 minutes for sintering.

Next, the sintered body was cut into a size of 35 mm×35 mm×5 mm, andsubjected to pack welding with a stainless steel SUS 304 sheet materialfor preventing oxidation during hot rolling. The cut-out piece washeated to about 800° C. by burner heating and subjected to hot rollingin a longitudinal direction as a sheet material at a rolling strain/passof 0.1 and a draft of 82%, and the pack material was removed. Thetitanium alloy composite material was charged into a vacuum furnace,subjected to vacuuming, and subjected to aging treatment at 500° C. for8 hours under an argon gas (133 Pa) replacement, to thereby obtain thetitanium alloy composite material of Example 4. Table 1 collectivelyshows the results of measurement of tensile strength, Young's modulusand hardness.

Example 5

The titanium alloy composite material of Example 5 was obtained in thesame manner as in Example 4 except that: the amount of the carbonnanotubes in the mixture of the titanium alloy powder and the Cr-coatedcarbon nanotubes was changed to 0.5% by mass; and the draft of hotrolling was changed to 81%. Table 1 collectively shows the results ofmeasurement of tensile strength, Young's modulus and hardness.

Comparative Example 1

50 g of the titanium alloy powder used in Example 1 was weighed andcharged into a graphite die of the pulse electric current sinteringdevice. The raw material powder was pressurized at 30 MPa with agraphite cylinder, depressurized to a degree of vacuum on the order of 4Pa, heated from room temperature to 900° C. with a temperature increaserate of 100° C./min, and maintained at 900° C. for 5 min for sintering.Next, the sintered body was cut into a size of 35 mm×35 mm×5 mm, andsubjected to pack welding with a stainless steel SUS 304 sheet materialfor preventing oxidation during hot rolling. The cut-out piece washeated to about 800° C. by burner heating, and subjected to hot rollingin a longitudinal direction as a sheet material at a rolling strain/passof 0.1 and a draft of 68%, to thereby obtain a titanium alloy compositematerial of Comparative Example 1. Table 1 collectively shows theresults of measurement of tensile strength, Young's modulus andhardness.

Comparative Example 2

The titanium alloy composite material of Comparative Example 2 wasobtained in the same manner as in Comparative Example 1 except that thehot rolling was omitted. FIG. 8 shows the results of material strengthmeasurement of the titanium alloy composite material of ComparativeExample 2. Table 1 collectively shows the results of measurement oftensile strength, Young's modulus and hardness.

Comparative Example 3

The titanium alloy composite material of Comparative Example 3 wasobtained in the same manner as in Example 2 except that the multilayercarbon nanotubes were directly used without Si coating. Table 1collectively shows the results of measurement of tensile strength,Young's modulus and hardness.

Comparative Example 4

The titanium alloy composite material of Comparative Example 4 wasobtained in the same manner as in Example 1 except that the hot rollingwas omitted. FIG. 8 shows the results of material strength measurementof the titanium alloy composite material of Comparative Example 4. Table1 collectively shows the results of measurement of tensile strength,Young's modulus and hardness.

The results revealed that the titanium alloy composite material of eachof Examples 1 to 5 had a tensile strength of 1,500 MPa or more and aYoung's modulus of more than 120 GPa, and thus had significantlyimproved mechanical strength than that of conventional titanium alloys(Comparative Examples 1 and 2).

Meanwhile, the titanium alloy composite material of Comparative Example4 produced by omitting the hot rolling, which means no carbon nanotubeswere dispersed in the crystal grains of the titanium alloy, had a lowtensile strength of 493 MPa, and thus had a mechanical strength moresignificantly degraded than those of the conventional titanium alloys(Comparative Examples 1 and 2). In the titanium alloy composite materialof Comparative Example 4, the carbon nanotubes or titanium carbonate waspresent on a periphery of titanium alloy fine particles like a shell ofa boiled egg, and served as the origins of cracks. Thus, sufficientmechanical strength presumably cannot be obtained.

The titanium alloy composite material of Comparative Example 3 employingthe carbon nanotubes without Si coating had mechanical strength moredegraded than those of the conventional titanium alloys (ComparativeExamples 1 and 2). In the titanium alloy composite material ofComparative Example 3, bonding between the titanium alloy and the carbonnanotubes was insufficient, and thus sufficient mechanical strengthpresumably cannot be obtained.

Evaluation of the mechanical properties of the titanium clad materialwas conducted following the procedure described below.

<Material Strength Measurement>

A target material was cut out into a dumbbell-shaped test piece having alength of 30 mm in a rolling direction, and parallel and perpendiculardirections, a length of a parallel part of 15 mm, and a width of theparallel part of 5 mm with a carbon dioxide gas laser. A strain gaugewas attached to the parallel part, and strength measurement wasconducted at a crosshead speed of 1 mm/min by using a material testingmachine (manufactured by Shimadzu Corporation, Autograph AG-1, 100 kN).

<Elongation Measurement>

A strain gauge was attached to the parallel part of the test piece ofthe target material through an adhesive, and a lead wire of the straingauge was connected to a bridge. Then, the whole was set in a materialtesting machine through a strain meter for elongation measurement.

<Fracture Toughness Measurement>

The fracture toughness measurement was conducted by a K_(IC) testingmethod in accordance with ASTM E399-90 or ISO 12737. Introduction of afatigue precrack and measurement of fracture toughness were conductedwith an electrohydraulic servo fatigue testing machine (MTS 810 TestStart II).

Example 6

The elongation and fracture toughness K_(IC) of the titanium alloycomposite material (thickness of 1.6 mm) obtained in Example 2 weremeasured. The elongation was 6%, and the fracture toughness K_(IC) was45.1 MPa·M^(1/2).

Next, the titanium alloy composite material of Example 2 and aTi-4.5Al-3V-2Mo-2Fe sheet material (available from JFE SteelCorporation, SP-700, thickness of 1.0 mm, subjected to solution agingtreatment at 510° C. for 1 hour) as a high toughness titanium alloy werelaminated into a graphite die of the pulse electric current sinteringdevice. The whole was pressurized at 30 MPa with a graphite cylinder,depressurized to a degree of vacuum on the order of 4 Pa, heated fromroom temperature to 950° C. with a temperature increase rate of 100°C./min, and maintained at 950° C. for 5 min for sintering, to therebyobtain the clad material of Example 6 containing the titanium alloycomposite material and the high toughness titanium alloy bondedtogether. This titanium alloy composite material had a tensile strengthof 1,425 MPa, an elongation of 9.7%, and a fracture toughness K_(IC) of50.4 MPa·m^(1/2). Meanwhile, the high toughness titanium alloy (i.e.,conventional titanium alloy) used above had a tensile strength of 1,213MPa, an elongation of 14.4%, and a fracture toughness K_(IC) of 55.8MPa·m^(1/2).

FIG. 11 shows a metallographic microscopic image of the vicinity of asinter bonded interface of the titanium clad material of Example 6, andFIG. 12 shows an enlarged image of an A part of FIG. 11. Themetallographic microscopic images suggest that in the titanium cladmaterial of Example 6, the titanium alloy composite material and thehigh toughness titanium alloy are favorably sinter bonded together.

The results revealed that the titanium clad material of Example 6contained the titanium alloy composite material and the high toughnesstitanium alloy favorably sinter bonded together, and thus had a tensilestrength of more than 1,400 MPa, an elongation of more than 9%, and afracture toughness of more than MPa·m^(1/2), which are mechanicalproperties more remarkably improved than those of the conventionaltitanium alloys.

1.-5. (canceled)
 6. A method of producing a titanium alloy compositematerial, comprising: mixing carbon fibers and a powder of an elementwhich forms a carbide in reaction with carbon, subliming the elementunder high temperature vacuum, and coating the carbon fibers with alayer containing the element and the carbide to produce coated carbonfibers; mixing the coated carbon fibers and titanium alloy powder toform a mixture, and applying a mechanical impact force to the mixture tofix the carbon fibers on the surface of the titanium alloy powder toobtain a carbon fiber-fixed titanium alloy powder; sintering the carbonfiber-fixed titanium alloy powder to form a sintered body; and plasticworking the sintered body to disperse the carbon fibers in crystalgrains of the titanium alloy.
 7. The method of producing a titaniumalloy composite material according to claim 6, further comprising agingthe plastic-worked titanium alloy composite material.
 8. The method ofproducing a titanium alloy composite material according to claim 6,including sintering is with pulsed electric current.
 9. The method ofproducing a titanium alloy composite material according to claim 6,wherein the plastic working is at least one process selected from hotrolling and isothermal forging.
 10. The method of producing a titaniumalloy composite material according to claim 6, wherein the elementcomprises at least one element selected from the group consisting ofsilicon (Si), chromium (Cr), titanium (Ti), vanadium (V), tantalum (Ta),molybdenum (Mo), zirconium (Zr), boron (B), and calcium (Ca).
 11. Themethod of producing a titanium alloy composite material according toclaim 6, wherein the carbon fibers comprise carbon nanotubes,vapor-grown carbon fibers, or a mixture thereof.
 12. The method ofproducing a titanium alloy composite material according to any claim 6,wherein the mixture of the carbon fibers and the titanium alloy powdercomprises 0.1% to 10% by mass of the carbon fibers. 13.-16. (canceled)